There is a plug on top of the transmission fluid reservoir (top, not the bottom). You have to remove that plug on top and add fluid through that hole. The vehicle must be level in both directions, engine running, in Park, parking brake set, and at operating temperature to get it right. If no fluid comes out or splatters out it is low. Add fluid until it begins to overflow. It makes it easy if you have a top with a squirt nozzle that will fit the fluid bottle or put the fluid into a plastic squeeze bottle with that type of top. Attach a piece of tubing (18 to 24 inches long) to the squirt top nozzle, put the loose end into the transmission hole, and squeeze the bottle to transfer the fluid. You can find this procedure on the internet if not enough information here.
You will add transmission fluid to a 98 Isuzu rodeo through the transmission overfills screw opening. Add the fluid until it flows out over the overfill screw opening, then close the overfill screw. To check the transmission fluid, have your local repair shop put the car on a lift and use the two check holes on the passenger side of the transmission case.